A method for the production of nickel metal powder from basic nickel carbonate by reduction with gaseous hydrogen at elevated temperatures and pressures is disclosed in U.S. Pat. No. 3,399,050 to D. J. I. Evans et al. The process utilizes a concentrated ammoniacal solution of nickel ammonium carbonate which is initially diluted with water and then boiled to remove excess ammonia and carbon dioxide. This results in the precipitation of basic nickel carbonate (BNC), i.e. a mixture of nickel hydroxide and nickel carbonate, leaving essentially no nickel ions in solution. This slurry is then charged to the autoclave, heated to temperature and reduced with hydrogen. The nickel powder is effectively formed by direct reduction of the solid BNC.
This prior an procedure deleteriously yields a powder containing some entrained, or encapsulated BNC, which results in a lower specific gravity and increased levels of oxygen and carbon which are unacceptable for certain applications. Additionally, the prior art process is difficult to control to yield consistent results, since the boiling step produces variable results.
The prior an process has always used a combination of ferrous sulphate and aluminum sulphate as the catalyst, but the iron content of up to 4000 ppm, or the high total metallic impurity (up to 0.8% ) in the nickel metal powder precludes its use in certain applications.
In the paper entitled "Effect of Addition Agents on the Properties of Nickel Powders Produced by Hydrogen Reduction" by W. Kunda, D. J. I. Evans and V. N. Mackiw in "Modern Developments in Powder Metallurgy. Vol. I: Fundamentals and Methods" Hausner, H, H, and Roll, K;. H. eds. (New York: Plenum Press, 1966), 15-49, there is detailed a discussion of a wide variety of alternative catalysts and additives and their effects in modifying the physical properties of the nickel powder produced.
During recent years, fine nickel powders have been produced commercially for use in electronic circuitry, fuel cells and numerous other usages. However, in certain specialized applications, exemplary of which are conductive pastes used in capacitors and the like, it has been found unacceptable to utilize the existing available nickel powders in such pastes because of the high level of impurities, for example, iron, alkali metals, carbon and oxygen which deleteriously affect conductivity. Thus, at present, the industry is using fine powders prepared from alloys of the platinum group metals, gold and silver in the formulation of such pastes. As will be readily appreciated, the smaller the particle size, the thinner the layer of paste which will be required for the substrate. Clearly, too, a spherical particulate configuration is sought after to thereby provide tighter packing concomitant with a layer of increased conductivity. Therefore, it is an objective of the present invention to provide an equally effective, but less costly replacement for the metals in current usage.
Additionally, it is an object of this invention to provide a process for preparing micron-sized, spheroidal nickel metal powder having higher purity, and a production process exhibiting improved reproducibility.